Use of ink-jet printing apparatus for creating color images based on digital image data is well-known. Basically, color ink-jet printing apparatus include printheads, or portion of a single printhead, each dedicated to placing ink (or, more generally, "colorant") at specific locations on a print sheet in accordance with the digital image data. In most basic digital color printers, the primary colorants available to be placed at various locations on the print sheet, which correspond to different types of signal accepted by the printing apparatus, correspond to the subtractive primary colors cyan, magenta, and yellow. Further, most commercially-available ink-jet printing apparatus further include a special printhead or portion of a printhead dedicated to placing pure black ink in desired locations on the print sheet, either as part of a full-color picture to be printed, or to print text. Less expensive types of color printing apparatus, in some circumstances, print a "process black" by combining the cyan, magenta, and yellow ink on the same location on the print sheet, the combination of these inks generally forming a black area.
A common concern in ink-jet printing is the deleterious effects of placing too much liquid ink in a given small print area on the print sheet. Placement of a large quantity of liquid ink on a specific location on a print sheet will cause the paper fibers on the print sheet to be oversaturated with liquid ink, and this oversaturation is the source of many possible print defects. For example, placing a large quantity of liquid in a small print area will require an appreciable amount of time for the liquid ink to evaporate out of the paper fibers; if the liquid ink does not evaporate from the paper fibers quickly enough, it is likely that the still-liquid ink will be smudged as the sheet moves further through the apparatus. Further, even if the liquid ink dries sufficiently, paper which has been wet and then dry will inevitably show "cockle," which is the bending and bubbling of paper caused by different portions of the paper drying unevenly. The problems of smudging and cockling occur even when very small amounts of liquid ink is placed on a print sheet, but the problems become particularly acute when a number of droplets of different primary colors are placed in close proximity on a print sheet in order to obtain a particular desired hue.
One possible way of minimizing the amount of liquid ink placed in a small area on a print sheet is to make available, in addition to the additive primary colors, a set of secondary colors which can be selectively placed on the print sheet, the secondary colors representing in effect "pre-mixed" combinations of the primary color. As it happens, these secondary colors are typically additive primary colors, such as red, green, and blue. By placing one of these secondary colors at a required location on the print sheet, one droplet of the secondary color can do the work of two droplets of primary colors, thus "saving" having to place three droplets of ink on a particular location.
Another design consideration known in ink-jet printing is to distinguish between two specific types of "black," either a "pure black" or a "process black." Even in printing apparatus in which a separate black in is available for printing text, in some situations this pure black, which is intended not to be mixed with other inks in close proximity thereto on the print sheet, is of a slightly different hue than would be a "process black" created by the intermixing of the primary color inks on the print sheet. A pure black, such as to print text, used to create dark areas in a photograph has been known to create a noticeable print artifact. Thus, it is a design option in ink-jet printing apparatus to distinguish between the requirement of a pure black, such as to print text by itself, and the process black, which would be used to create, for example, dark areas in reproducing a color photograph.
Given the considerations of using secondary colors to avoid having to place large amounts of liquid on small areas in the print sheet, and also the desire to make available both pure black and process black ink, an effectively "eight-color" ink-jet printing apparatus can be imagined. This printing apparatus would include not only the availability of subtractive primary colors, cyan, magenta, and yellow (CMY) but also the secondary colors red, green, and blue (RGB) as well as a pure black K.sub.1 and a process black K.sub.2 which is a pre-mixed set of the CMY colorants, which will fit in with images created with the CMY colorants. It is thus possible to create a CMYRGBK.sub.1 K.sub.2 eight-color digital printing apparatus. Given this availability of hardware, consideration must be given to the fact that, in most available image-rendering schemes, every full-color image comprises only four primary color sub-images, CMYK (with no distinguishing between process black and pure black). It is thus a function of the present invention to provide hardware in which original CMYK image signals can be converted for use in an eight-color CMYRGBK.sub.1 K.sub.2 eight-color printing apparatus.